This invention relates to a method for removing mercury from dilute sulphuric acid. According to the method sulphuric acid solution is scrubbed with an aqueous solution containing thiosulphate, which precipitates the mercury present in the sulphuric acid solution either as mercury sulphide or, in a chlorine-containing environment, as a mercury sulphide-mercury chloride double salt. The quantity of thiosulphate to be fed is adjusted so that it precipitates the mercury in the sulphuric acid solution, but still does not form elemental sulphur.
Sulphuric acid is generally manufactured from sulphur dioxide-containing gases such as zinc and copper concentrate roasting and smelter gases. The gases are routed to scrubbing towers, where they are scrubbed with dilute sulphuric acid with a concentration in the region of 35-45 wt %. Mercury is removed from the gases as thoroughly as possible, but some droplets of dilute sulphuric acid solution used in gas scrubbing, with an Hg content of up to 10-20 g/l, also leave in the gas flow from the scrubbing towers. This is equivalent to 7.7-15.3 kg/t H2SO4 in sulphuric acid with a content of 40 wt %. The proportion of mercury carried in the droplets may be many tens of times greater than gaseous mercury. The thorough removal of mercury from the circulation acid droplets carried along in the gases is therefore crucial in the preparation of top-quality sulphuric acid.
U.S. Pat. No. 6,228,270 describes a method whereby calcium, mercury and selenium are removed from very dilute sulphuric acid used in gas scrubbing. The sulphuric acid content is only 0-50 g/l. Impurities are removed from the solution before it is neutralised and sewered. Calcium is removed from the solution by filtration before the other substances are removed. Mercury is removed from the filtered solution by precipitation with sodium sulphide and selenium is removed by means of sodium dithionite. The precipitated impurities are removed by filtration in the same stage.
GB patent 1409456 describes a method where mercury is removed from concentrated sulphuric acid by adding thiosulphate to the acid and separating the sulphur and other precipitate generated for instance by flotation or filtration. According to the method, thiosulphate always decomposes first into sulphur trioxide and elemental sulphur. The method is based on the principle that as thiosulphate breaks down, the elemental sulphur generated is colloidal and highly reactive, and that it has a wide and active surface. The mercury dissolved in the concentrated sulphuric acid reacts with this colloidal sulphur precipitate. After some time, the sulphur particles agglomerate to a particle size of about 100 μm. The mercury in the acid is precipitated with the sulphur. It is advantageous to feed plenty of thiosulphate (5-10 kg/m3 acid), since it accomplishes the rapid precipitation of sulphur agglomerates. Elemental sulphur and mercury are preferably removed from the sulphuric acid by flotation, where a frother is fed into and air is routed into the acid. The sulphuric acid described in the patent is the end product of an acid plant, concentrated sulphuric acid, of which the lowest mentioned content is 70% and the amount of soluble mercury to be removed is quite small, with an initial content of a maximum of about 25 g/t H2SO4.
The mercury removal method presented in GB patent 1409456 is based on the decomposition of thiosulphate in concentrated sulphuric acid and the reactivity of the sulphur formed. Thiosulphate is fed abundantly in relation to the amount needed to bind the excess mercury. The text mentions as an example, that there was 3.5 g Hg/t H2SO4 in the sulphuric acid and that 1 kg of thiosulphate was fed per cubic metre of H2SO4. As the acid content of the sulphuric acid in this case was 70 wt %, this means that 1 kg of thiosulphate was fed per 1.611 t H2SO4 (=621 g/t H2SO4). Expressed as a molar ratio this means that the thiosulphate/mercury ratio is about 140, when the thiosulphate formula is Na2S2O3.5H2O. The precipitating sulphur is removed by flotation, but this is rather a difficult treatment method for concentrated sulphuric acid. The other method mentioned in the example is filtration.